A question that I was asked that Im unsure about....can a company do their own fixed electrical inspection testing using one of their own electricians (17th Edition)? If this can be done in-house, what specific training is needed to allow a spark to carry this out? Initially I was of the thought that this must be done externally, but Im not so sure now.

If the Mods don't delete this post! There is no need for 2391, the requirement is for competence in this work. This is much more than just being a "qualified spark". 2391 is part of the way toward proving this. The inspector must be familiar with the kind of installation such that he can comment on its condition in comparison with the existing regulations, and in the previous versions back until it was installed, as this will determine whether the installation requires updating. Mere non compliance with current regs does not mean the install requires updating, nor does it mean the install is unsafe.